ERBIL, Iraq, Feb 11 (Reuters) - Western-backed Syrian forces should isolate Islamic State's de facto capital in Syria "by the spring" before an offensive on the city itself, British Defence Secretary Michael Fallon said on Saturday.

The Syrian Democratic Forces, which includes the powerful Syrian Kurdish YPG militia, launched the campaign on Raqqa in November.

It announced this month the start of a new phase in the offensive, aiming to complete its encirclement of the city and cut off the road to the militants' stronghold in Deir al-Zor, southeast of Raqqa.

"I hope that isolation will be completed by the spring and then operations to liberate Raqqa itself can begin thereafter," Fallon told reporters in Erbil, the capital of the Kurdish autonomous region in northern Iraq.

Islamic State is fighting hard to preserve its foothold in Syria as it loses ground in Iraq.

U.S.-backed Iraqi and Kurdish forces last month dislodged the militants from the eastern side of Mosul, their last city stronghold in Iraq, and are preparing an offensive on the parts of the city that lie west of the Tigris river.

"Raqqa is a much smaller city than Mosul but will clearly be defended very vigorously by Daesh and that means the operation to liberate Raqqa has to be very carefully prepared, as the operation for Mosul was," Fallon said, using an Arabic acronym for Islamic State.

"Once Raqqa is liberated after Mosul, we will see the beginning of the end of this terrible caliphate," he said. Islamic State declared the caliphate in parts of Syria and Iraq in 2014.

Britain is part of the U.S-led coalition supporting forces battling Islamic State in both Iraq and Syria.